Telephone system



ay 3-. c. E. BEALE ET AL TELEPHONE SYSTEM 14 Sheets-Sheet 1 Filed Sept. 28, 1931 TELEPHONE SYSTEM 14 Sheets-'Sheet 2 Fi led Sept. 28, 1931 May 30,- 1933. c. r- BEALE El AL TELEPHONE SYSTEM '14 Sheets-Sheet 3 Filed Sept. 28, 1931 M y 30, 1933- c. E. -BEALE El AL TELEPHONE SYSTEM 14 Sheets-Sheet 4 Filed Sept. 28, 1931 i E a TELEPHONE SYSTEM Filed Sept. 28, 1931 14 Sheets-Sheet 5 le 4e u M u. Tv 1 5 mmv A H 1 m mTo NWT. 2 1 In 2 F. W W. I W M 1 M .u 4 a k A [0 am 2 1 M I 0 m 2 I m JLIIII.

y 1933- c. E. BEALE El AL 1,912,122

TELEPHONE SYSTEM Filed Sept. 28, 19 31 14 Sheets-She et 6 y 1933- c. E. BEALE El AL 1,912,122

TELEPHONE SYSTEM Filed Sept. 28, 1931 14 Sheets-Sheet 7 y 1933- c. E. BEALE ET AL 1,912,122

TELEPHONE SYSTEM Filed Sept. 28, 1931' 14 Sheets-Sheet 8 May 30, 1933.

c. E. BEALE ET AL TELEPHONE SYSTEM Filed Sept. 28, 1931 14 Sheets-Sheet 9 m M MQXQ X y 1933- c. E. BEALE ET AL TELEPHONE SYSTEM Filed Sept. 28, 1931 14 Sheets-Sheet 10 y 30 1933 c. E. BEALE ET 111. 1,912,122

TELEPHONE SYSTEM 14 Sheets-Sheet 1i Filed Sept. 28, 1931 N qq g :3 w Q AH .a Q 1 2 5 Mr, u u

May 30, 1933.

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May 30, 193 0-5 BEALE ET AL TELEPHONE SYSTEM Filed Sept. 28, 1931 14 Sheets-Sheet l3 y 1933- C.-E. BEALE ET AL TELEPHONE SYSTEM Filed Sept. 28, 1931 14 Sheets-Sheet 14 Patent ed May 30,

UNITED STATES) cmnas PA ENT OFFICE rimmNn nnann AND REGINALD news, or :arvnaroon, ENGLAND, assIeN- ORS TO ASSOCLATED TELEPHONE AND TELEGRAPH OF CHICAGO, ILLmOIB, A. CORPORATION OF rnLnrrroNn sYs'rim Application fled September 28, 1881, Serial No The present invention relates to telephone systems and more particularly to arrange-- ments whereby automatic switches may be controlled by equipment provided in common to a grou of switches.

According to one feature of the invention in a group of automatic switches assoclated with common controlling apparatus, the extension of the speaking loop to one of the said switches causes the operation of. two relays in series over the loop, one of said relays being located in the common controlling apparatus, while the other isindividual to the switch.

According to a further feature of the invention a'te ephone system comprises a plurality of groups of lines, each group being served by regular and overflow finders in which the regular and overflow finders are brought into use over difierent wipers of a further switch.

According to another featureof the invention an impulse repeater is associated with common controlling apparatus which apparatus is adapted to be associated with a repeater by theoperation of a relay individual to there ater.

Accor ing to yet another feature of the invention a group of final selector switches are adapted to co-operate with incoming nu-' merical switches associated withv common controlling apparatus at a distant exchange over the a two wire junction and are provided with means for placing a busy condition on? the unctions associated with numerical switches when thecommon apparatus associated with said switches is taken into use. 1 g. Y

The invention will be better understood from the following description of apreferred embodiment, reference being had to the accompanylng drawings.- 7

F1 1, 2, 3 and 4 show the der switches, v v Fig. 5 shows a first group selector with two sets of wi rs, I 1 1 Fig. 6 s owe a second group selector with one set of wipers, 1

Fig. 7 showsa second group selector'with two sets of wipers,

Fig. 8 shows a final selector with two sets of w1pers, a

arrangement of 565,605, and in Great Britain October 2, i830.

Figs. 9 and 10 show a P. B. X. final selector, Fig. 11 shows an impulse repeater, and Fig. 12 shows a la out dia am, while Figs. 13 and 14 s ow a discriminating selector repeater for use with incoming numerical selectors associated with common controlling apparatus.

The common,equipment is preferably associated with a vertical group of switches, one controlling the first selector on each of say ten shelves, another controlling the second selectoron each of say ten shelves, etc. This method has the advantage over the arrangement of associating the equipment with a horizontal group of switches in that each shelf of ten selectors has access to ten difier are connected together, the finder switches being controlled by common equi ment which is associated wi h the switch to e controlled by means of a distributor switch whilst the selector-switches are controlled by common equipment which is common to all the switches in a group. The two grou s of common equipment are interconnected y operatingrcircuits extending from one to the other.

he first selector switch includes other featuressuch as arrangements for double hunt .ing with preferential switching, for givingoverflow busy from the common equipment,

and in the case of permanentloo s or releasing the common equipment an connectback number 'unobtainable tone from the sefiactor. The final selector switch is also provided with two sets of wipers, but in this case the set of wipers taken into use de ends upon the route over which it is seized. inging, metering, and busy tone are connected upin the common equipment. The brid e relays for supplyingspeaking current to t e callin and called .sjubs'criber's are not connecte 'up untilthe calledsubscriber replies butfare'thenconnected by a relay'in the switch which relay is hel operated dependent upon the calling loop. I

of 200 and each grou In the repeater the bridge placed across the outgoing loop after an impulse train has terminated 1s located in the common control equipment and can consequently be-of low impedance, thus avoiding the tendency of giving an extra impulse to the line relay at the distant end of the trunk line which has existed inprevious re aters.

Moreover the impu se repeatin relay is located in the common control equipment so that this relay can be made of low impedance for responding accuratel to impulses whilst the relay in the repeater or supplying s aking current can be made of high imped nce so as to reduce transmission loss.

Furthermore the common control equi ment is adapted to be disconnected from t e repeater both when a called subscriber replles and when a called operator replies, diferent signals being received by therepeater ,in the two cases.

Figs. 1, 2, 3 and 4 show a line finder of known type except that overflow finders are incorplorated therewith and minor modifications ave been made for working with common control group selectors.

Subscribers lines are divided into groups is provided with a number of individual nders and when these are all bus overflow finders are brought into use. uflicient individual finders are provided to cater for all traflic except the peak, during which the overflow finders are rought into use. Two, three, four or five over ow finders, each one from a difierent 200 line group, are connected to one group I selector and a number of similar combinations will constitute an overflow grou This overflow grou caters for the tra eaks from several different 200 line groups 0 subscribers. The peaks of trafic from say four 200 line groups will not coincide and advantags is taken of this to save an a preciable number of first selectors. The num r of different line finders connected to one overflow group selector is determined by the trafic. The trunking schematic diagram shown iii Fig. 12 illustrated this feature.

In this figure ARD and AOD illustrate the two wipers of the distributor switch over which the regular overflow finders respec- Y tively are brought into use. The wiper ARD is connected to the regular finders such as lARF and 2ARF which in,turn are directl connected to the first group selectors lAFG and QAFGS associated with common controlling apparatus lFGC and 2FGC respectivegroup selectors" '3BF G S," 3CFGS -and 3D GS are similarly connected ya overflow finders from-other the selector only is held. The pro osal is to step the selector and hunt for a ree N. U. tone equipment on level 1. It is proposed to fit one common equi ment for tenfirst selectors. The overflow usy signal is given from the common control equipment, because this condition should not afiect more than 1 call in 500, with the standard grade of service.

The second gr up selectors may be of the 10/10 or 10/20 t pe. The associated common equipments wi 1 not contain relays for throwin .back P. Gs, but overflow busy will be given from them. It is proposed to fit one common equipment for 20 selectors. Similar type selectors on any rank are, of course, interchangeable. K

The regular final selectors are of the 200 line type and rovide battery feed for the calling and cal ed parties. Rin 'ng, metering, and busy tone are controlled y the common equipment. As the common equipment is held until the called party answers, it is pro osed to fit one common group for six fina selectors.

The P. B. X. final selectors may have from 2 to 10 lines in groups and employ a common control equipment from which is given P. B. X. hunting facilities, ringing, metering, and busy tone. As the common equipment is held until the called party answers, it is roposed to .fit one common equipment for six final selectors.

Repeaters for use between two automatic exchanges fitted with common control simplify the following problems which exist in the resent design of re eaters ((1% Introducing a hig impedance bridge across the out oing 103p after an impulse train is finishe has ten ed to given an extra im ulse to the distant A relay.

- F6) The local A relay should have low impedance for dialling and high impedance for supplying speaking battery.

(0) Difliculty of giving supervision when the call is extended to a manual board, since reversing battery sets up metering conditions.

The proposed circuit shows how these problems are met in a simple manner. At the same time operating limits wider than are at present ssible are provided. As the common equipment is held until the called party answers, it is roposed to fit one common equipment for our repeaters.

Referring now particularly to Figs. 1, 2,

3 and 4 in which Figs. 2 and 4 represent a subscribers line circuit, a line .finder and a distributor associated with the so-called A grou and Figs. 1 and 3 represent similar circuits associated with the B group.

When a subscriber in the group originates a call relay LR will operate from battery, lower winding of relay LR, armature 002, conductor 10, circuit containing calling subscribers line and instrument, conductor 11, armature 003 to earth and at armature 1.7-3 applies earthto the combined start common and level marking wire on the finder vertical bank, at armature Zrl, earth is also applied to the private normal conductor 42 to;

guard the line against incoming calls. Relay A in the distributor is therefo'reenergized, the circuit being battery, windin of relay A, armatures krl, 2115, ya2, non-in uctive resistance 5YA, armatures cal and M3 to earth.

The distributor rotary line switch is preselecting and operates in the following manner. If a contact on the regular guard bank, that is, the bank with which wiper 7a engages is earthed, relay S of the distributor operates in series with relay H but without operating relay H, and at armature 81 completes a circuit for the distributor driving magnet DDM which in operating at interrupter springs dd'm. breaks the circuit of relay S. This interaction between relay S and the driving magnet continues until a free contact on the regular guard bank is reached. It should be mentioned here that relays YA. and ZA in the A group distributor circuit,

Figs. 2 and 4, and relays lYA and 1ZA in a the B group distributor circuit, Figs. 1 and 3, are normally operated and for this rea- Q son their armatures are shownon the drawings in anzoperated position. It will be seen that relay YA will only remain operated as long as there is a disengaged finder in the A group so that if all the finders are busy,

relay YA will release since earth will be removed from all conductors such as conductor 16, and at armature g a2swill transfer the start common to the 1A relay in the B group of finders.- If all the finders. in the B group are busy, the start common is transferred back to the relay A of the A group by the release of relay IYA in the B group distributor circuit and the overflow finders are brbught into use.

Assuming now that a disengaged finder is encountered in the A group, wiper 7a will now no longer find earth on the bank contact but battery via resistance YO in the first group selector, .-.Fig. 5. Thus relay Hwill o erate over the circuit: batte resistance C, 5, armatures 27:61, 2 2,busying jack 3T1 of the first-group selector, private conductor.15, busying jack T1 of the finder switch, contacts N3, bank contacts and wiper 7a of the distributor switch ADS, armatures ya3 and k1, bothwindings of relay H in series, armatures I04 and alto earth. Current will also flow. from battery,- winding of relay S, interrupter springs ddm, armature hl, lower winding of relay H, armatures M and al to earth but the resistance of the lower winding of rela H is such as to prevent the o eration of re ay S in this circuit. Relay operates and locks over its upper lowresistance winding via armatures b2 and k1 in series with the resistance YO in the first group selector, and at armature k3 energizes relay K which operates-and locks via armatures kand a1. At armature 106 a circuit is completed for relay 2A in the first group selector common equipment, Fig. 5 which therefore operates from dial tone battery, armature b122, winding of relay 2A, strap X, armatures 2hb4 and 272415" in the first group selector, negative talking conductor 13, armatures M5 and hb5 in the finder switch, contact and wiper 6a of the distributor switch, armatures fr2, 01, g1 and 106 to earth. At

the winding of relay FR ofthe distributor and in addition the circuit for the vertical magnet VM of the finderis' also completed from battery winding of the vertical magnet VM, ba contact and wiper 3a of the distributor ADS, low resistance winding of relay Gigrmatures cl, 1 and k6 to' earth. It

should noted that t 'e first selector shown in Fig. 5 is adapted for. use either in association with a finder switch as described or as an incoming selector; When used in the first capacity strap Y is removed and strap X is added while in the second capacity strap X is removed and stray Y is added. Relay FR alsooperates in series with relay 2A of the first group selector common equipment and at armature {r1 breaks the circuit of the'win'ding of re ay C. When the vertical magnet o e ates its contacts B'will operate relay G o the distributor from battery, winding of relay G, armature fr3, wiper 2a and bank contac I of switch ADS, vertical magnet interrupter springs, contacts V to earth and at armature? 91 willbreak the circuit of the vertical ma et and relay C. This type of interaction etweeu the relay and the magnet isvwell-known to those skilled in the art and it'will continueuntil the finder has raised its wipers to the marked level, whereupon the earth which has been applied to the level marking wire at armature Zr? in the subarmature 2a1 relay 2B is energized and in The same 'earth Y armature 01 the short circuit is removed from scribers line circuit will now be transmitted NR1 bank contact and wiper 1a of switch ADg, armatures c2 and g2, winding of relay G, resistance YA to battery, thus maintaining relay G operated. Relay C which has hitherto remained operated-over its low resistance winding in series with the vertical ct will now release since its circuit is bro an at armature g1 so that relay G will also release when its circuit is broken at armature 02.

The rotary magnet RM of the line finder is now operated from battery, winding of rotary magnet, bank contact and wi er 4a of the switch ADS, low resistance win ing of relay KR,'armatures #2, 01, g1 and 106 to earth and will step the wipers of the finder switch into engagement with the first set of bank contacts on the particular level, at the same time connecting relay G via rotary magnet springs R and armature ha2 to the finder switch wiper 3c. Relay KR will also operate over this circuit and lock up over armature {M2 to earth at ofl-normal spring contacts N3.

Assuming that the line circuit associated with this first set of contacts is not that of the calling subscriber the contact upon which wiper 3c is resting will be earthed, either via armature 112 in the line circuit if the particular line is disengaged or if the line be engaged via the private wiper of the line finder which is bein used on that call. In either case this earth will serve to operate relay G which at armature 91 will break the rotary magnet circuit. This interaction between relay G and the rotary magnet will continue until the contacts associated with the calling line are reached.

If it be assumed that these contacts are now in engagement with wipers 1c, 20 and 30 relay HA will now switch during the time that the rotary magnet springs R are closed the circuit being, earth, armature kr3, wiper.

la and bank contact of. the switch ADS, lower winding of relay HA, rotary magnet springs R, bank contact and wiper Qu of switch ADS, armature fr3, winding of relay G to battery, but the resistance of this winding of HA is such as to prevent the operation of relay G in this circuit. At ar-' mature 7M2 earth on the private conductor 15. is now extended by wiper 3c and bankcontact of the finder switch and armaturev Z7'2 of the calling subscribers line circuit to operate nelay CO over its lower winding and also to hold relay LR over its upper winding. Relay CO at armature 001 disconnects. the earth from the start common wire which releases relay A in the distributor, thus ini-. tiating the release of all distributor-relays but the first group selector associated with the finder switch will now be held under the control of the calling subscriber, since the circuit for relay 2A in, the first group se lector common equipment will now be dial It will be seen that at the time when re-' lay HA of the finder switch is 0 rated the corresponding winding of relay B will be short circuited on one side by the earth via armature 1: 13 and wiper lo, and bank contact of switch ADS and on the other side by the earth which:will be connected to that contact on which wiper 60 is now resting since the line associated with this contact is not the calling line. If, however, the callin line is associatedwith the level with whic wipers 4c, and engage then relay HB will switch and relay HA remains short circuitcd, Refay HR in switching performs exactly the same functions as relay HA except that the other set of line wipers are extended to the first group selector.

When the conversation is finished, the switching relay HA or HB, as the case may be, released and a circuit is completed for the release magnet of the finder switch from battery which may be via the well known release alarm relay, winding of release magnet ZM, armatures M23 and M13, ofi normal contacts N1, armature 706 'of the distributor switch to earth in order to restore the finder switch to normal.

If a fault causes the start relay A of the distributor switch to remain operated for longer than some predetermined period (usually three seconds) the time pulse which operates in conjunction with relays TA and negasubscribers forming the A group on the upg per five levels.-

As has previously been stated, when all finders in the A group are busy, relay YA of the A group distributor circuit will re- 7 leaseand at armature 3 112 will transfer the start common wire to relay 1A in the B group distributor circuit. Relay 1A will therefore be operated on subsequent calls ori inated in the A group,and the finder in the p group will therefore carry all traific while the re ular A finders remain busy, but should a the B finders also become busy relay lYA will restore. and transfer the A group start common back to its own distributor. Further calls originated in the A group will now be dealt with by the overflow group of finders but if these also become busy the relay ZA will release so that in the event of other calls being originated during this period, relay LO will be energized to operate the lost call meter (not shown). The sequence of operations is identical for calls originated in the B group except that the procedure is reversed. V 7

Referring to the subscribers line circuit Fig. 2, when the subscriber originates a call his meter will be short circuited by earth on the private conductor via armature c04, and the upper winding or relay CO, but when the call is finished a local circuit may be traced as follows, earth, winding of the subscribers meter, armature e04, upper winding of relay CO, armature lr2, upper winding of relay LR, lower winding of relay CO to battery.

This circuit is of high resistance and the meter conse uently does not operate. Also in this condition the windings of relay CO will now beacting in opposition, so that it quickly releases, thus breaking the-already mentioned local circuit. When the booster battery is applied to the rivate conductor for metering purposes the s ort circuit on the meter will thereupon be replamd by full booster battery (which, as is well-known, is usually a 50 volt battery with its negative ole earthed) and the meter will then operate.

en booster battery is replaced by guardin earth again the meter will release and it wil be'seen therefore that several applications of booster battery to the rivate conduc-' tor which may conceivably ta e place in zone metering areas will give .a corresponding number of registrations on the subscribers meter. While booster battery is connected to the rivate conductor the two windings of -relay 0 will assist each other; they oppose only during the releaseof the connection; If zone metering is not required the line circuit shown' can, of course, be simplified considerably. Referring now to the first group selector shown in Fig. 5 when the 'subscribers loop is extended from the finder switch relay 2K will operate over the circuit previously outlinedand will lock up over its right handwinding from battery, right hand winding of relay 2K, make first armature 2101, armature 262 to earth. It will be remembered that relay 2A and consequently relay 2B are already operated by the earthsent forward from the finder distributor. Relay 2B, by earthing the common private at armature 2b3, busies all first selectors in the group.

The first impulse from the subscribers dial circuit completed for the switchin 'EHA, armature 2106, rota releases relay 2A which will step the vertical magnet 2VM of the first oup selector over the circuit, battery, winding of the magnet 2VM, normal rotar springs 2NB1, armatures 271:5 and 2g1,r1ght hand low resistance winding of rela 2C, armatures b123, 2126 and 2a1 to earth; lay 2C also operates over this circuit and, in the well known manner remains operated durin the train of impulses. It should be pomted out that only the vertical magnet of the group selector taken into use is o erated as in all associated selectors the 2K relay is not ener gized.

. When the vertical impulses are finished relay 20 will release and complete a circuit for the rotary magnet 2RM of the first group selector from battery, winding of the rotary magnet 2RM, ofi' normal contacts 2N3, armatures 2104 294, bbl and 201 to earth which will therefore step the wipers of the Clip selector into engagement with the banl f both of the private contacts with which wipers 3e and 6e engage are earthed, relay 2G will operate when the rotary magnet springs 2R make near the end of the rotary step the circuit being battery, right hand winding of rela 2G,rotary magnet springs 2R, armature 2 a3, wiper 36 to earthed private contact. Further, the other coil ofrelay 2G will be short-circuited, on one side by the other earthed contact via wiper 6e, armature 2kb2, normal; rotary springs 2NR1, armatures 21:5 and 291, and on the other side earth-via armatures 201, 661 and 292 so t a t although relay 2G will operate in order to break the rotary magnet circuit at armature 294 yet its left hand holding winding will be inoperative. This interaction between relay G and the. rotary magnet will continue until a free trunk to the next rank of switches, characterized by an unearthed private contact, is found.

If the wipers 3e and 6e encounter disengaged contacts simultaneously then as the rotary magnet is energized and the rotary magnet springs 'R are closed there will a re a 2HA from-earth, lower winding 0 rela; magnet springs 2R, right hand windin o relay 2G to battery. Rela 2HA will ock operated over its upper win ing to the earthed private conductor via armature 2ha2, but relay 2G will not be able to operate owing to the comparatively highresistance of the lower wind- I mg of the relay 2HA. The negative and positive talking conductors and the private conductor are switched to the next selector ahead and the common equipment of the first oup selector will be released since the circuit for relay 2A is now'broken at armature 2M5.

If thev private contacts associated with wiper 36 were busyand that associated with relay 2HB, armature 2107, normal. rotary to battery in order to step the wipers one springs 2NR1, armatures 265, and 291, resistance D to battery. Relay 2HB will look over its left hand winding to the private conductor via armatures 2661 and 271.02 in order to switch the appropriate talking conductors and private conductor to the next selector rank. g

If all trunks on the particular level dialled be engaged, the switch wipers will be rotated to the eleventh rotary position whereupon the cam springs CS will be operated. Relay 2G will operate via cam springs CS1 and rotary magnet springs 2B and relay BB will then operate via cam springs CS2 and armature 2g3 thereupon locking independent of armature 293 via armature 664. The. earth via springs CS1 will also be extended to the eleventh step private contact to which is connected the overflow meter for that particular selector level, so that this meter will be operated in order to register the overflow condition. At armature 662 busy flash is connected to the calling conductors via the winding of relay 2A and a circuit is completed for connecting busy hold battery to the winding of relay 2B via armatures 663, 266 and 2a1 which will hold relay B operated each time the circuit of its left hand windin is broken due to the intermittent release 0. relay 2A in response to the busy flash.

If a subscriber fails to dial within a predetermined plied by the S pulse cam will 0 erate relay I TP over its right hand winding via armature 264 and. cml so that this relay will operate and lock over its left hand winding from battery, left hand winding of relay TP, armatures tpl, 0:02 and 263 to earth. At armature tp2 the relay TZ is connected to the Z pulse cam 'so that upon the lrext operation of this cam it will supply eartlrto operate relay TZ which at armature $22 will complete the vertical magnet circuit from earth, armatures tz2, 663, right hand winding of relay 2C, armatures 291 and 2705, normal rotary sprin s 2NR1, winding of the vertical magnet 2VI vertical step. Upon the cessation of the Z pulse, the vertical magnet and relay 20 will release and the rotary cut-in of the wipers on the bank will take place, the switch thereupon hunting for a free number unobtainable tone equipment connected to the contacts of the first level.

Referring now to Figure 6, this shows a second group selector of the lO-level 10-trunk type and it is assumed that the call is extended from the'first selector to a group selector of this type.

When the subscribers loop is extended from period the time pulse earth sup-' the first selector, relays 3K and 3A will operate in' series over the circuit, earth, left hand winding of relay 3K, armature 2711, conductor 46, circuit com rising subscribers line and instrument via t e first group selector,

the line finder'switch and the line circuit, conductor 45, armature 262, strap 1X, winding of relay 3A, armature 1661 to battery, relay 3K thereupon locking up to the earth sent forward on the private conductor 47, via itsown make first armature 3101. Relay 3A completes a circuit for relay 3B via armature 3a1 which operates-and at armature 361 busies all associated selectors, and at'armature 362 holds relay K. The train of impulses constituting the next dialled digit will step the switch wipers to the desired level after which relay 3G will release and allow the rotary magnet 3RM to step the wipers into engagement with the bank. If the first trunk is busy relay 3G will operate and break the rotary magnet circuit and this interaction will continue until a free trunk is found whereupon the characteristic absence of earth on the contact with which wiper 3/ will now be resting will enable relay 2H to switch in series with relay 3G the circuit being, battery, winding of relay 3G, rotary magnet springs 3R, armature 3706, lower winding of relay 2H to earth. When relay 2H operates it switches through to a final selector such as the one illustrated in Fig. 8, and releases the common equipment. If all trunks on the level dialled are busy the wipers will be rotated to the eleventh rotary position on that level and the cam springs operated to register on the overflow meter at cam springs lCSl and to o erate relay lBB at springs 1CS2. Relay 1B in operating at armature 1661 will connect busy flash to the calling line via the winding of relay 3A and at armature 1664; apply the busy hold condition to the right hand winding of relay 3B. It should also be noted that in this circuit the switch magnets are all associated with the common apparatus over off-normal 'contacts.

Fig. 7 shows a first group selector of the 10-level 20-trunk type the circuit operations of which are similar to those of the first group selector shown in Fig. 5, with the exception that there is no dial tone equipment or facility for the transmission of number unobtainable tone if the subscriber delays to dial. A detailed description will, therefore, not be given of this circuit.

Referring now to the final selector shown in Fig. 8, it will be seen that either relays WS and 4A or relays 4K and 4A will operate in series over the calling subscribers loo dependent upon the selector level from whic the connection is extended, this final selector being of the type which is commonly referred to as a 200 line final selector. If relay'WS operates the switch will function over one group of 100 lines whereas if relay WS does 

